The present invention relates to frequency sweeps, and, more particularly, to a multi-arm frequency sweep generator operating at radio frequency.
Many communication devices utilize frequency sweeps, e.g., demodulators, swept receivers and spectrum analyzers. A frequency sweep is a waveform with a monotonic frequency function of time. Typically, linear frequency sweeps are desired.
Frequency sweeps can be generated by time dispersing a pulse as a function of frequency. The pulse is selected to have frequency components over the desired output range of the sweep.
A reflective array compressor (RAC) is one device used to produce such dispersion. The RAC may be manufactured by etching plural slits in a large quartz crystal. The size and placement of the slits determine the dispersion characteristics of the RAC. For example, a RAC can be designed to provide a linear or other specific sweep. RACs are well-known in the art and are described for example in "The Use of Surface-Elastic Wave Reflection Gratings in Large Time-Bandwidth Pulse Compression Filters", R. C. Williamson et al, I.E.E.E. Transactions on Microwave Theory and Techniques, Vol. MTT-21, No. 4, April, 1973, pp. 195-205.
In one application, a surface acoustic wave (SAW) demodulator can incorporate three or three sets of RACs. The input to the demodulator can be a frequency division multiple access (FDMA) signal. For each symbol or bit period of the FDMA signal, a first or input RAC time-staggers the individual frequency bands that make up the multiplexed signal. A second frequency sweep RAC converts pulses, synchronized to the input bit periods, to sweeps. The sweeps are combined with the staggered frequency channels to produce a series of sweeps. A third or output RAC compresses these sweeps into pulses which represent the time division multiplexed (TDM) decoding of the FDMA input.
While RACs perform quickly and reliably, they are difficult to manufacture precisely. A RAC designed to produce a linear frequency sweep, for example, will generally have some characteristic non-linearities. The cost of manufacturing each RAC typically prohibits large scale rejection of deviating devices. In addition, the characteristics of a RAC can change due to temperature. For RACs to be used in environments such as space with temperature extremes, it is desirable to be able to calibrate a RAC to maintain linearity.
In many applications, an error signal can be obtained which when zeroed indicates a linear frequency sweep. More specifically, a sweep can be applied to a known input signal, and the result compared to the expected output. In the SAW demodulator described above, a constant known input should produce a series of pulses with very little spreading. Any spreading can be analyzed as a function of frequency to identify points of non-linearity in the frequency sweep and allow for correction.
However, in applying corrections device encounter performance constraints which limit the slope and repetition rate of sweeps that can be corrected reliably. Device technology continues to ease these limits, but desired objectives remain distant. What is needed is an approach that can greatly extend the frequency slopes and repetition rates that can be reliably corrected with current technology.